Switch rails are present at all railway turn-outs, and allow trains to change tracks. This is achieved by having an extra pair of rails present—the switch rails—which can be moved from a position where they do not interfere with the standard rails and the train wheels, into a position where the train wheels are diverted on to one or more of the switch rails, and so the train is diverted onto the turn-off. The radius of the turn off required (and hence length of the switch rails) varies depending on a number of factors including the terrain and the speed of the train at that section of track.
The switch rails are required to move within strict dimensional limits. Often, the tip of the switch rail is driven into position by a point machine located between, or at the side of the tracks, level with the free end of the switch rails. The force required at this point is exponentially proportional to the length of the switch rail. To ensure the switch rail is located within the required dimensional limits, a series of linked cranks is often employed—with a crank being attached to the switch rail at regular intervals along its length and mechanical linkages connecting all the linked cranks.
In most current linked crank designs, power is transferred from the output of the point operating machine, via the switch rail, to the linked cranks. The point machine moves the tip of the rail, which is connected to a first linked crank, which is thus rotated. This first linked crank is connected to the rest of the linked cranks, and so they all rotate increasingly smaller amounts to ensure the switch rail is located properly. This means that the point machine needs to provide power not only to move the tip of the switch rail, but also all of the linked cranks. This adds significant additional load to the point machine, increasing wear and causing increased incidences of breakdown.
There are new, alternative point machines which address this unreliability somewhat. However these have been expensive to produce, and require a significant amount of work to install, making it expensive, time-consuming and slow to update a significant number of turn-outs with these alternative systems.